The present invention generally relates to noise reducing apparatuses for reducing noise by compression and expansion of signal levels, and more particularly to a noise reducing apparatus capable of improving noise modulation phenomenon introduced as a secondary effect.
Conventionally, noise reducing apparatuses have been known which perform compression of the signal level and expansion of the signal level complementary to the signal level compression, in order to reduce noise (hiss noise and the like) introduced upon reproduction of a magnetic recording medium in a magnetic recording and/or reproducing system. These known noise reducing apparatuses were used for reducing noise (surface noise and the like) introduced upon tracing of a record disc during reproduction in a recording and reproducing system for the record disc, and further used for improving the degradation introduced in the ratio between receiving signal level and receiving noise level (signal-to-noise ratio) upon long-distance signal reception and the like in the field of signal transmission and reception of radio broadcasting.
In the above conventional noise reducing apparatus, a level compression circuit compresses the signal level of a signal which is to be transmitted, and a level expansion circuit expands the signal level of an incoming level compressed signal obtained through a transmission path by a quantity corresponding to the level compressed quantity of the incoming level compressed signal. Moreover, control voltages of respective variable emphasis type gain control circuits in the above signal level compression circuit and the signal level expansion circuit, are produced by giving predetermined time constants to envelope wave detected outputs of output signals or input signals of the above variable emphasis type gain control circuits. Accordingly, since the control voltage level varies according to a level expansion characteristic corresponding to the level variation of the level compressed signal, the noise within the level compressed signal applied to the signal level expansion circuit is subjected to level modulation by the signal level expansion circuit. Therefore, noise modulation phenomenon was introduced in the output of the signal level expansion circuit.
Hence, in the above conventional noise reducing apparatus, degradation is introduced in the auditory fidelity if the level modulated noise obtained from the signal level expansion circuit is not masked by the signal level.
The above noise modulation phenomenon is closely related to the returning time of the signal level expansion circuit. Generally, when the returning time is long, the noise modulation phenomenon is easily notable and unpleasant to the ear, and on the other hand, when the recovery time is short, the noise is masked and hardly detected by the ear. Accordingly, the noise modulation phenomenon can be improved by reducing the above recovery time. However, there is a limit in reducing the above recovery time, since it is necessary to prevent the distortion factor in the level compressed and expanded signal from becoming worse due to a ripple component included within the control voltage in the lowermost signal frequency at which the signal compression and expansion is performed. Therefore, there was a disadvantage in the conventional noise reducing apparatus in that it was difficult to greatly improve the noise modulation phenomenon.